The use of pavement markings (e.g., paints, retroreflective elements, tapes, and individually mounted articles) to guide and direct motorists traveling along a roadway is well known. During the daytime the markings may be sufficiently visible under ambient light to effectively signal and guide a motorist. However, the degree of conspicuity or visibility depends in large part on the pavement surface. For example, a white pavement marking on a concrete road may be difficult for the motorist to see because of the lack of contrast.
In addition, at night, especially when the primary source of illumination is the motorist's vehicle headlights, the markings may be insufficient to adequately guide a motorist because the light from the headlight hits the pavement and marking at a very low angle of incidence and the light is not sufficiently reflected back toward the motorist. Thus, improving daytime conspicuity and night time retroreflection is desirable.
Retroreflection describes the mechanism where light incident on a surface is reflected so that much of the incident beam is directed back toward its source. The most common retroreflective pavement markings, such as lane lines on roadways, are made by dropping transparent glass or ceramic microspheres or optical elements onto a freshly painted line such that the optical elements become partially embedded therein. The transparent optical elements each act as a spherical lens and thus, the incident light passes through the optical elements to the base paint or sheet striking pigment particles therein. The pigment particles scatter the light redirecting a portion of the light back into the optical element such that a portion is then redirected back towards the light source.
In addition to providing the desired optical effects, pavement markings must withstand road traffic, adverse weather conditions, and cost constraints.
Pavement marking articles and other substantially horizontal markings typically exhibit high retroreflective brightness when the light is incident at high entrance angles (typically greater than about 85.degree.). Retroreflective sheeting and other retroreflective articles attached to vertical surfaces, on the other hand, tend to exhibit high retroreflective brightness at lower entrance angles (e.g., within 30.degree. to 40.degree. of normal). Thus, the optics of pavement marking articles differ from the optics of retroreflective sheeting.
The retroreflective efficiency of flat pavement marking articles is limited because the exposed surfaces of the optical elements are directed upward, whereas the optimal orientation is toward vehicle headlights which typically illuminate the retroreflective beads from angles slightly above the road surface and because the optical element alignment results in the exposed surface of the optical elements being exposed to maximum abrasive wear by vehicle tires. Pavement marking articles having protrusions have several advantages including, runoff of rain water and availability of locally non-horizontal surfaces to support optical elements.
The need exists for substantially horizontal retroreflective pavement marking articles having enhanced visibility and contrast.